In many film application, such as packaging of foodstuffs, chemical and hazardous materials and in medical applications, the industry requires films having certain properties. In the packaging of foodstuffs, for example, the films must have high puncture resistance, high clarity and gloss, and reduce permeability to gases and/or vapors. The films used to manufacture containers for chemicals and hazardous waste materials must have high puncture resistance, high elongation strength, high tear resistance and chemical resistance. Films used in medical applications, such as blood bags, must have a high puncture resistance, low modulus, high tear resistance and auto clavability.
Polypropylene provides a combination of desirable properties, such as tolerance of high temperature, retardation, or prevention, of moisture vapor transmission and approval for contact with food and medical solutions. Thus, polypropylene has been widely used in film and sheets for packaging of foodstuffs, chemical and hazardous materials and medical applications. However, polypropylene does suffer from relatively poor brittleness, low impact strength and clarity.
Attempts have been to overcome the shortcomings of polypropylene in films and sheets by modifying the polypropylene itself or by coextruding the polypropylene with other polyolefin type resins. For example, U.S. Pat. No. 4,514,534 discloses a modified polypropylene comprising low molecular weight polypropylene, a peroxide, a nucleating agent and linear low density polyethylene for biaxially oriented films to improve clarity.
U.S. Pat. No. 4,950,720 discloses modified polypropylene comprising a reactor blend of a high molecular weight propylene/olefin random copolymer with a lower molecular weight substantially isotactic propylene homopolymer. The reference teaches that the modified polypropylene having a molecular weight distribution of from about 5 to about 10 can be extruded or coextruded for use in film applications.
In U.S. Pat. No. 5,011,719 are disclosed multilayer films comprising at least two layers of modified polypropylene-based compositions comprising a blend of a propylene-ethylene copolymer, an ethylene-butene copolymer and optionally an elastomer in either or both layers.